It is common to use a feedback circuit with amplifiers, from the point of view of performance and design considerations. Amplifiers feed loads that could include an external capacitor, and it is known that these external capacitors generally have equivalent series resistance (ESR) values. The capacitor ESR impacts the performance of the associated circuitry or impacts the associated design considerations.
The ESR of an external capacitor which interacts with an amplifier or regulator might introduce a zero in the loop transfer function. Even though the zero in the loop transfer function might result in improved phase margin, there may be stability-problems because of the wide variation of the ESR. The ESR of external capacitors of the type referred to, especially if they are ceramic capacitors together with other parasitic resistances, might vary from 1 milliohm to 300 milliohms. Such variation of ESR makes compensation very difficult. Typically to reduce the impact of the ESR variation, one can add an intentional internal resistance of about 50 milliohms. Such addition of intentional internal resistance helps to reduce the order of ESR variation by about 50 times (to 50-350 milliohms only), thus making the design easier. The addition of the intentional resistance however causes the load regulation to degrade. Since the added intentional resistance is usually a metallic resistor, it is usually temperature-sensitive and exhibits a +−20% variation owing to process and 4300 ppm with temperature, making the 50 milliohms look like a much larger resistance at higher temperatures and a lower one at lower temperatures. To compensate for the low temperature value, if the nominal value of the resistance is increased, load regulation will have to be sacrificed even more, especially at high temperatures. It is not easy to optimize performance with such constraints for high bandwidth/high current amplifiers.
Proposed herein is a topological design modification which would not hamper stability for capacitor ESR variations and not degrade load regulation in an amplifier.